This invention relates generally to the control of unwanted seepage in below-ground environments, and more particularly is directed towards isolating unsaturated brines from mining passageways.
Underground liquid seepage in mines and other below ground level situations has been a continual problem since the time mines and other underground storage facilities were first created. Numerous attempts have been made to seal off passageways and prevent the seepage of unwanted liquids. This problem is most acute where the host rock is an incompetent or soluble rock such as that found in salt rocks or near potash deposits.
One method used to prevent this seepage was to pour in, through a suitable entry method, aggregates of fly ash and cement to seal the passageway. Nevertheless, a certain amount of high pressure unsaturated brines and other unwanted liquid mobile substances travelled through the microfractures of the surrounding host rock and seepage continued.
More recently, in the art of isolating unwanted liquids underground, U.S. Pat. No. 3,925,992 issued to Backstrom for a method and reservoir for storing nuclear residues in an underground rock cavity reservoir comprised of surrounding a container for the resudues with a viscous liquid such as bitumen or Bentonite having a density somewhat above that of water which has sealing action on cracks in the container and/or the surrounding rock formation. The container is connected to the walls of the rock formation. The container is connected to the walls of the rock cavity by means of resilient or flexible supports. The aforementioned patent is somewhat successful in dealing with underground seepage of pressurized liquids but has not eliminated the problem.
The problem is particularly acute in deep potash mines located in Canada, where flooding has occurred in salt rocks consisting of more than one mineral such as sylvite, carnallite, and halite. Even at very low pressures, unsaturated brine will bypass an engineered barrier through the cracks and fractures in the wall of the mine opening adjcent to it. In addition to bedding planes and other lithological discontinuities there are fractured zones in the area subjected to stress concentrations during and after the excavation of an opening.
Prior to constructing a barrier, all fractured and otherwise potentially discontinuous material must be removed. However, even a complete removal of all such material will not prevent infiltration of brine into the stress relieved zones around the opening. Scanning electronic microscope photographs of potash specimens have shown that stress relieved potash rock is full of microcracks resulting from the difference in strains of the sylvite and halite crystals. Therefore, it has now become known that a successful design of the isolation structure should not only contain an impermeable seal at the interface of the rock and the dam or barrier, but should also prevent migration of the brine through microfracture systems in the vicinity of the structure.